A toroidal continuously variable transmission for a vehicle disclosed in U.S. Pat. No. 4,434,675 and U.S. Pat. No. 5,136,890 varies a speed ratio in proportion to the hydraulic pressure supplied by a control valve.
The speed ratio is fed back to the control valve through a mechanical feedback device.
In addition, U.S. Pat. No. 5,674,145, U.S. Pat. No. 5,669,845 and Tokkai Hei 8-326887 published by the Japanese Patent Office in 1996 disclose an electronic feedback controller wherein a target speed ratio is set according to the running state of the vehicle, and the control valve is controlled so that a real speed ratio of the continuously variable transmission is equal to this target speed ratio.
The operating characteristics of the continuously variable transmission and the mechanical feedback device change according to various conditions such as an output shaft rotation speed.
gyration angle of the power roller which transmits a torque inside the continuously variable transmission, a line pressure supplied to the control valve and the temperature of hydraulic fluid which is a medium for transmitting hydraulic pressure.
Therefore, it is desirable to set the gain of the feedback control carefully according to these conditions in order to achieve a stable speed ratio control.